The present invention relates to processes for decolorizing alkanolamines and alkyleneamines, and more particularly, to both batch and continuous processes for producing alkanolamines and alkyleneamines having improved color characteristics by treatment with solid acidic catalyst.
Price and purity are important characteristics of a chemical's marketability to a potential customer. During industrial processes used to produce various chemicals, impurities often enter the processes and manifest themselves in the form of color contamination. The sources of this color contamination may be metals and metal compounds from the catalysts or equipment used in the processes, or conjugated organic compounds which are formed during the processes. Since the mechanism by which these color contaminants are formed varies from process to process, the decolorization process normally acts by a mechanism which depends on the color source.
It is well known that alkyleneamines, particularly the higher polyamines, become discolored during their preparation which generally reduces their commercial value. Various procedures have been used commercially or suggested for decolorizing or otherwise improving the color of these compounds including, for example, treating with hydrogen chloride or amine hydrochlorides as disclosed in JA-119902, UK Pat. No. 1,351,050; treatment with activated carbon, acid treated clays or acidic zeolites at elevated temperatures (about 200.degree. C.) as disclosed in U.S. Pat. Nos. 3,723,529 and 4,737,243; and treatment with a sulfonic acid ion exchange resin as disclosed in U.S. Pat. No. 4,731,165. In each of these procedures, distillation is generally a final step needed to achieve the desired color, and neutralization of residues, handling and disposal of acids, or reactivation of the treatment medium require special apparatus which complicate the processes.
U.S. Pat. No. 4,570,019 discloses a process which may be run continuously for producing polyalkylene polyamines having improved color characteristics which comprises treating discolored polyethylene polyamines with polyethylene polyamine hydrochloride in the presence of water at elevated temperatures, thus eliminating extra handling steps described in the processes above.
However, as is known in the art, methods used for decolorization of higher polyethylene polyamines are generally not transferable to compounds such as alkanolamines which are prepared by different processes and which use different reactants.
For example, alkanolamines may be conventionally prepared by reacting an alkylene oxide with ammonia or an amine, or, more particularly, alkanolamines such as aminoethylethanolamine may typically be prepared by the reductive amination of monoethanolamine.
Alkanolamines produced by these conventional processes can vary in color from nearly colorless water-white liquids to pale yellow. Alkanolamines, particularly ethanolamines, are susceptible to color formation in the presence of oxygen (e.g. from air leaks during manufacture and/or storage), excessive temperature and soluble metals such as iron or nickel. Also, impurities in raw materials may contribute to the initial and continuing formation of color bodies.
Various attempts to overcome the problems associated with color and color instability of alkanolamines have proven to be cost intensive and environmentally unacceptable. Such attempts have included purification by fractional distillation, concentration of color forming bodies or their precursors by fractional distillation, carbon treatment, the use of adsorbent materials, various hydrogenation techniques, and the use of reducing agents such as sodium borohydride and hydrazine.
U.S. Pat. No. 2,744,938 describes a process for the treatment of color-sensitive organic compounds which have become discolored through oxidation or upon aging. The patent specifically describes the method of removal of color bodies from alkylphenols with a catalyst-free solid adsorbent in the presence of hydrogen maintained at super atmospheric pressure. The thus treated alkylphenol is then separated from the solid adsorbent material.
U.S. Pat. No. 3,207,790 describes a process for improving the color of alkanolamines that develop undesirable color on aging by adding a sufficient amount of a borohydride of an alkali metal. The borohydrides have appreciable solubility in the alkanolamines and after dissolution, may remain in the solution or the alkanolamines may be distilled under reduced pressure.
U.S. Pat. No. 3,453,183 describes a method of removing aldehyde impurities from ethanolamines by forming a mixture of the ethanolamines with either powdered silicas, powdered silicates, liquid silicates or powdered aluminas, followed by separation of the ethanolamines by vacuum distillation.
U.S. Pat. No. 3,819,710 describes a process for improving color and color stability of ethanolamines by hydrogenation using selected catalysts and selected catalysts conditions. Useful catalysts for the process include Raney nickel, platinum, palladium or ruthenium.
EP 0477593 describes purification and decolorization of off-color crude N-dialkyl dialkanolamines by vacuum distilling in the presence of water and a water-soluble metal borohydride.
The above patents confirm the need for methods of improving the color characteristics of both alkanolamines and alkyleneamines. While some of the processes such as the use of solid adsorbents results in improved color, such processes are not entirely suitable for large scale decolorization and require labor intensive steps for recovery of the desired product adding expense to the process in terms of time of treatment, equipment costs, and disposal of byproducts.
Other methods, such as hydrogenation of color impurities requires expensive Raney nickel which must be replaced on a regular schedule. Hydrogenation systems also require special equipment for hydrogen supply, mixing and, after reduction, filtration to remove the Raney nickel, since distillation in the presence of Raney nickel tends to generate color and deactivate the catalyst.
The need exists for an inexpensive easily operated process of producing alkanolamines or alkyleneamines having reduced color.